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Mechanism of Burning of Fully-Developed Compartment Fires Harmathy, T

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Mechanism of Burning of Fully-Developed Compartment Fires Harmathy, T NRC Publications Archive Archives des publications du CNRC Mechanism of burning of fully-developed compartment fires Harmathy, T. Z. This publication could be one of several versions: author’s original, accepted manuscript or the publisher’s version. / La version de cette publication peut être l’une des suivantes : la version prépublication de l’auteur, la version acceptée du manuscrit ou la version de l’éditeur. For the publisher’s version, please access the DOI link below./ Pour consulter la version de l’éditeur, utilisez le lien DOI ci-dessous. Publisher’s version / Version de l'éditeur: https://doi.org/10.4224/40001768 Paper (National Research Council of Canada. Division of Building Research); no. DBR-P-777, 1978 NRC Publications Archive Record / Notice des Archives des publications du CNRC : https://nrc-publications.canada.ca/eng/view/object/?id=da59c99e-b492-48f3-b564-98ba22d9577b https://publications-cnrc.canada.ca/fra/voir/objet/?id=da59c99e-b492-48f3-b564-98ba22d9577b Access and use of this website and the material on it are subject to the Terms and Conditions set forth at https://nrc-publications.canada.ca/eng/copyright READ THESE TERMS AND CONDITIONS CAREFULLY BEFORE USING THIS WEBSITE. L’accès à ce site Web et l’utilisation de son contenu sont assujettis aux conditions présentées dans le site https://publications-cnrc.canada.ca/fra/droits LISEZ CES CONDITIONS ATTENTIVEMENT AVANT D’UTILISER CE SITE WEB. Questions? Contact the NRC Publications Archive team at [email protected]. If you wish to email the authors directly, please see the first page of the publication for their contact information. Vous avez des questions? Nous pouvons vous aider. Pour communiquer directement avec un auteur, consultez la première page de la revue dans laquelle son article a été publié afin de trouver ses coordonnées. Si vous n’arrivez pas à les repérer, communiquez avec nous à [email protected]. er National Research Conseil national k Council Canada de recherches Canada N21d 0. 7777 K PUB MECHANISM OF BURNING OF FULLY-DEVELOPED COMPARTMENT FIRES s by T.Z. Harmathy Reprinted, from Combustion and Flame Vol. 31, No. 1, 1978 p. 265 273 d::! d::! q up,; 8.- DBR Paper No. 777 k&-J I Division of Building Research Price 25 cents OTTAWA NRCC 16722 SOMMAIRE La croyance selon laquelle un manque d'air limite la vitesse de d6veloppement d'un incendie 2 ventilation contr816e est trPs rhpandue. Apres examen critique, ce principe ne tient pas. D'autres m6canismes qui pourraient expliquer la faqon dont le dhbit de l'air dans un compartiment contrsle la vitesse de combustion (pyrolyse) d'un mathriau combustible cellulosique sont examinhs et un modsle plausible est prGsent6. L'hypothsse sugg6r6e est que l'oxydation de la surface carbonis6e est le lien qui explique le rapport observg entre le taux de pyrolyse et le taux d1entr6e d'air. Le modsle proposg permet de tirer des conclusions compatibles avec les connaissances actuelles sur les incendies I ventilation contr816e et ceux 2 surface de combustible contrCl6e. Une tentative d'explication du m6canisme de combustion I 11int6rieur d'une enceinte de polym6res synthstiques non carbonisables est 6galement donn6e. Mechanism of Burning of Fully-Developed Compartment Fires T. Z. HARMATHY Fire Research Section, Division of Building Research, National Research Council of Canada, Ottawa, KIA OR6 It has been widely believed that in ventilation-controlled building fires a shortage of air limits the rate of burning. This concept is critically examined and proved untenable. Other possible mechanisms by which the flow rate of air into a compartment may control the rate of burning (pyrolysis) of cellulosic fuel are ex- amined and a plausible model presented. It is suggested that oxidation of surface char is the link by which the rate of pyrolysis is related to the rate of entry of air. The proposed model provides conclusions compat- ible with present knowledge of both ventilation-controlled and fuel-surface-controlled fires. It is emphasized, in conclusion, that the model is probably not applicable to compartments in which the fuel consists pre- dominantly of non-charring plastics. INTRODUCTION combustion of volatiles (a series of gas-phase reactions), and (iii) the oxidation of the solid That the rate of burning of cellulosic fuel in fully- decomposition product, char (a heterogeneous developed compartment fires is, under a wide solid-gas reaction). On the other hand, what fire range of conditions, approximately proportional scientists refer to as rate of burning is, in fact, to compartment ventilation [I, 2, 3, 4, 5, 61 the rate of loss of mass of the fuel associated was probably the first important discovery of fire primarily with the departure of the volatile decom- science. The explanation seemed quite straight- position products and, to a lesser degree, with the forward. The rate of entry of air, it was believed, oxidation of char. This has nothing to do with limits the extent to which oxidation reactions can the rate of the dominant combustion process, approach the stoichiometric relations and, in the combustion of the volatiles, although this turn, limits the rate of heat evolution in a com- process is the chief source of heat evolution in partment. This view, apparently, still persists. the compartment. Clearly, it would be justified Several years ago the author [7] pointed out to replace the expression rate of burning by the that referring to poorly vented fires as "ventila- more appropriate (but still not fully accurate) tion-controlled" is indeed very apt, since the rate term rate of pyrolysis or rate of volatile produc- of entry of air literally controls, not limits, the tion. rate of burning; more exactly, it controls the rate- Although for some fuels (among them all determining link in a complex process hidden cellulosic materials) the nature of the decomposi- behind the simple word burning. tion reactions depends on the presence or absence What, in everyday language, is referred to as of oxygen in the atmosphere, the rate of pyrolysis ~urningis, with cellulosic fuels, a process consisting is controlled largely by the rate of heat supply 3f three entirely different types of reaction: to the decomposing fuel. Consequently, the rate 3) the pyrolysis of fuel into (roughly) 85% volatile of entry of air into the compartment can control md 15% solid decomposition products, (ii) the the rate of pyrolysis (the so-called rate of burning) Eopyright 01978 by The Combustion Institute published by Elsevier North-Holland, Inc. 0010-2180/78/0031-0265 $1.25 T. Z. HARMATHY only in an indirect way: by being instrumental entry of air and drives the pyrolysis reactions2 I in regulating the rate of heat supply to the fuel. by thermal feedback. Yet this suggestion will not Various mechanisms by which ventilation may stand scrutiny either. One must realize that the control the rate of pyrolysis of cellulosic materials gaseous phase in a burning compartment does not will be critically surveyed and a plausible model consist of a well-stirred mixture of volatiles, air, suggested. The mechanism of pyrolysis (burning) and combustion products; in other words, that of non-charring plastics will also be hypothesized. the conditions in it are far from ideal for gas- phase reactions. In fact, as dlbe discussed later, the flame envelope fairly well divides the gas- CRITICISM OF SOME SIMPLE CONCEPTS filled space into two zones, one consisting mainly Traditionally, the dependence of rate of burning of air at virtually undepleted oxygen content, on compartment ventilation has been explained the other mainly of volatile decomposition by claiming that the rate of entry of air sets an products and combustion gases. Consequently, upper limit to which oxidation reactions can it is as a rule the rate of entrainment of air into the approach the stoichiometric relations. That this flame envelope, not the rate of entry of air into view is untenable can be proved by analysing a the compartment, that controls the rate of com- multitude of compartment burn-out experiments. bustion of the volatiles and thereby possibly An equation developed by Thomas et al. (See Eq. affects the decomposition of the fuel. (2i) in Ref. [5]) reveals that in fully developed, Admittedly, increased ventilation does create ventilation-controlled fires the ratio of the rate improved turbulence along the flame boundaries, of entry of air to the rate of volatile production enhancing the rate of air entrainment into the is approximately 5.41. The author [7] found an flames and, in turn, the rate of combustion of the even higher ratio of rate of air flow to rate of volatiles. Yet any suggestion that the rate of volatile production, 6.13. In comparison, the combustion of the volatile decomposition products stoichiometric air requirement for the volatile might have more than a marginal effect on the decomposition products of a "typical" wood rate of pyrolysis of the fuel can be dismissed, for is only about 4.19 kg air per kg volatilesl. Con- the following reasons. sidering that under fuel-surface-controlled condi- The rate of combustion of the volatile decom tions (to be discussed later) the ratio of air flow position products has a decisive influence on t rate to rate of volatile production is necessarily temperature of the gases and compartment boun higher than 5.41, one can state with fair certainty aries during fire. As heat supply to the decom that the rate of entry of air is sufficient for the posing fuel is expected to take place by hig complete combustion of cellulosic materials within temperature-dependent transfer processes, one the boundaries of the compartment. In spite of this, the appearance of large flames outside the There are contradictory views on whether the pyrol windows of burning compartments is common.
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